Sunday, November 2, 2014

Wacky Weather with a Wishy Washy Forecast

During the Utah Snow and Avalanche Worshop (USAW) yesterday, a storm was raging in the mountains.

But that's Sugar Mountain, North Carolina, not Alta, UT. Actually, it's been a pretty crazy couple of days across much of the southeast with widespread cold weather and the earliest snow (traces) ever observed in Charleston and Columbia, South Carolina.

Meanwhile in Utah, we set a record high of 77ºF at the Salt Lake City International Airport on Halloween, the latest that 77ºF has ever been observed. Even with yesterday's frontal passage, the pre-frontal high of 72ºF was only 2ºF below the record high for the date of 74ºF.

The culprit in this wacky weather is a highly amplified upper-level pattern. The dynamic tropopause (basically jet-stream level) analysis for 0000 UTC 1 November (late on Halloween, Oct 31) illustrates this highly amplified pattern with a deep upper-level trough along the west coast of North America, a high-amplitude ridge over the central interior, and a deep trough over the east.

This highly amplified pattern resulted in the record high at the Salt Lake City Airport and the cold, snowy weather in the southeast.

With the trough now moving across northern Utah, temperatures have dropped considerably. Alta-Collins observed 2 inches of snow with the frontal passage late yesterday and, as of 9 am MST, has picked up another 4 inches or so from snow showers overnight and this morning. During that period, the've benefited from a slow moving precipitation band that continues to sit over them in the southwesterly flow.

Mountain snow showers will continue at times today and tonight, potentially with some lake effect in the late night and early morning hours. As usual, the where, when, and how much regarding the lake effect is uncertain. My best guess is that we'll see another 3-5" at Alta-Collins before things wind down, but there's the possibility of more if the band currently over them lingers or the lake gets going and they are in the cross hairs. I wish I could say more, but this is a wishy-washy forecast as these smaller scale precipitation features are hard to pin down.

2 comments:

I have a question regarding GSL surface area and its effect on lake effect snowfall. After three straight below average snowpacks in the Wasatch, the lake seems noticeably lower than it was after 2010-2011. With how shallow the GSL is, I'd assume that small changes in lake level would have major effects on total surface area and in turn, how much lake effect snowfall we get. Is there any study you know of that discusses the correlation between lake level and lake effect snowfall?

This is examined in http://journals.ametsoc.org/doi/abs/10.1175/JAMC-D-12-077.1 and covered in Chapter 5 of my book Secrets of the Greatest Snow on Earth.

Year-to-year variations in the frequency and intensity of lake effect are driven predominantly by meteorology. As a result, a big lake does not ensure a big lake effect year, nor does a small lake ensure a small lake effect year.

All else being equal, however, given a lake-effect event, some events would probably be bigger with a larger lake, others perhaps not much different. In other words, sometimes size matters, sometimes it doesn't. Much would depend on the characteristics of the event. In addition, as the lake shrinks, the change is largest across the lake rather than along it. Thus the impact of size changes could vary depending on flow direction. We haven't really done the work to understand these sorts of intricacies.

Further, none of our forecast models even come close to adequately incorporating the effects of the Great Salt Lake or *reliably* predicting lake-effect events. This is true even in research simulations at very high resolutions (we're close to submitting a paper on this).

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The Wasatch Weather Weenies discuss the weather and climate of the Wasatch Front and Mountains, western United States, and beyond.

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